Il contributo della geologia alla comprensione dei terremoti

Low-enthalpy geothermal reservoirs in the buried thrust front of the
Apennines: exploration in the alluvial plain south of Parma
Key words: Geothermal reservoir, Miocene turbidites, Parma
alluvial plain, thrust tectonics.
INTRODUCTION
The growing green economy stimulates scientific research in
the field of renewable energy resources; among these is
geothermal energy, which is becoming more and more
competitive even at low-temperatures (> 40°C). On the mountain
slopes and in the southern alluvial plain of the Po River basin
there is no high-enthalpy geothermal source known at
economically convenient depths. Yet, thermal springs and wells
are numerous in the Emilia-Romagna Apennines and alluvial
plain. For instance, a geothermal reservoir at Ferrara, represented
by fractured Mesozoic limestones situated at 1,100-1,900 m
depth, delivers 120 l/s of water at 95-100°C temperature; at
Bagno di Romagna (Forlì-Cesena) thermal springs at 42-43°C
temperature have historically been used for balneotheraphy and
lately for heating too.
The screening of the Emilia-Romagna territory for potential
geothermal resources completed by the Regional Geological
Survey led to the recognition of several areas with positive
thermic anomalies. In general their geographic distribution
coincides with the location of the main tectonic structures. In the
subsoil of the Po River alluvial plain these structures are
represented by a system of faulted folds named Emilian Folds by
PIERI &GROPPI (1981). The Emilian Folds actually represent
thrust fronts whose crest may be encountered at a few tens of
meters depth below ground surface, as in the case of the alluvial
plain south of Parma which is the target of this pilot study.
STRATIGRAPHY AND THRUST TECTONICS
The southern margin of the Po River plain represents the
perisutural zone of the Apennines orogenic belt in which
_________________________
(*) Dipartimento di Scienze della Terra, Università di Parma,
nicola.calda@unipr.it
(**) Servizio Geologico Sismico e dei Suoli, Regione Emilia-Romagna,
FMolinari@Regione.Emilia-Romagna.it .
NICOLA CALDA (*) & FABIO MOLINARI (**)
719
Neogene and Quaternary sedimentation has been controlled by
thrust tectonics. The study area, in particular, is characterized by
a buried thrust front deforming Miocene-Pleistocene beds
(CALDA et alii, 2007). The Pliocene-Quaternary sedimentation,
modulated by eustatic sea-level fluctuations, has been interpreted
on the basis of regional unconformity surfaces (RER &ENI-AGIP,
1998) in alluvial sedimentary synthems (Qc2 and Qc1 of Late-
Middle Pleistocene age) and marine sedimentary synthems
(Costamezzana and Stirone of Early Pleistocene - Late Pliocene
age and Lugagnano and Vernasca of Middle - Lower Pliocene
age).
The geometry of the top of the marine deposits
(Costamezzana synthem), dated at about 800 ky BP, is presented
in the form of the Digital Surface Model of Fig. 1. The several
hundreds of depth values of this surface have been transformed
into elevations referred to sea level which have been spatially
correlated according to the kriging square-net method by means
of the ESRI software Spatial Analyst.
The prominent feature of the kriged map is the positive
structure, extending south and west of Parma on the alignment of
the towns of Monticelli Terme, Vigatto, Lemignano Madregolo,
Fontevivo and Fontanellato, representing a thrust generated
antiform with a modest synform on its southern limb and a steep
monocline dipping northeast. At Monticelli Terme the alluvial
deposits are directly superposed on the eroded Miocene substrate
and are only 10 m thick. Westwards the thickness of the alluvium,
modulated by the culminations and depressions of the positive
structure (Fig. 1), gradually increases and its erosive base overlies
the marine Costamezzana synthem.
GEOTHERMAL RESERVOIRS
The Emilian Folds have been extensively investigated as
potential oil and gas reservoirs with the production of a rich
wealth of data, in particular, seismic profiles, stratigraphic wells,
and numerous well-log data. Additional information, with precise
depth-temperature profiles, is provided by the so-called UNMIG
collection and the ENEL-CNR studies on geothermal resources.
The implementation of these data with the databases of the
University of Parma and the Regional Geological Survey allowed
the construction of key geologic cross-sections and their dressing
with the temperature changes at depth. The positive thermic
SESSIONE 23